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A surge protector is an appliance designed to protect electrical devices from voltage spikes. Surge protectors attempt to regulate the voltage supplied to an electric device by either blocking or shorting to ground voltages above a safe threshold. An outlet strip is sometimes miscalled a surge protector, but provides no such protection.

Over time, surge protectors lose their ability to protect against power surges and begin to basically function as an outlet strip.

Contents 1 Important Specifications 2 Primary Components 3 Other Important Components 4 See also 5 External links

Here are some specifications which are critical to understand when choosing a surge protector.

• Clamping voltage - This says what voltage will cause the metal oxide varistors (MOVs) to conduct electricity to the ground line. A lower clamping voltage indicates better protection. There are three levels of protection in the UL rating -- 330 V, 400 V and 500 V. For most purposes, a clamping voltage more than 400 V is too high.

• Energy absorption/dissipation - This rating, given in joules, says how much energy the surge protector can absorb before it fails. A higher number indicates greater protection. Look for a protector that is at least rated at 200 to 400 joules. For better protection, look for a rating of 600 joules or more.

• Response time - Surge protectors don't kick in immediately; there is a slight delay as they respond to the power surge. A longer response time means your computer (or other equipment) will be exposed to the surge for a greater amount of time. Look for a surge protector that responds in less than one nanosecond.

• surge current (kA)

The principal components used to reduce or limit high voltages usually includes one or more of the following electronic components:

• Metal Oxide Varistor - The metal oxide varistor (MOV) is a device containing a material that conducts current (shorts) when presented with a voltage above its rated voltage. MOVs can prevent high voltages from continuing at full levels to the susecptable circuits. MOVs typically limit voltages to about 3 to 4 times the normal voltage in the circuit. MOVs have finite lifetime, and will "wear out" when exposed to a few very large transients, or many more smaller transients. MOVs are used in parallel to increase current capabilty and lifetime. The normal failure mode is a short circuit, so MOVs usually are fused or otherwise protected when used in an ac power application. Some surge protectors have a light indicating when this stage has been reached. MOVs are the principal voltage limiter in most ac power protectors.

• Transient Absorption Zener - This is a zener diode designed to protect against voltage spikes in a circuit. These components provide the best limiting action of protective components, but tend to have the lower current capability. These typically limit voltages to 1.5 to 2 times the normal operation voltage of the circuit. If the current impulses are within the rating of the device, lifetime is exceptionally long. However, if the ratings of the component are exceeded, they can fail, usually as a short circuit. Due to the relatively limited current capacity and the exceptional clamping, transient absorption diodes are usually used in circuits that have limited exposure, or these devices are protected by higher capacity components. These are sometimes paired for bi-polar operation as a transient voltage suppression diode, and sometimes put in series to increase capability.

• Gas Discharge Tube (GDT) - These rely on a gas trapped between two electrodes to become ionized by the high voltage, and conduct electrical current. GDTs can pass much more current for their size than other components. GDTs also have a finite life time, and can take a few very large transients or a greater number of smaller transients. GDTs also take time to trigger and can let a small part of the voltage spike get past the component. It is not uncommon for a GDT to let through pulses of 500V or more of 100ns in duration. In some cases additional protection is necessary to prevent damage due to this effect. GDT provide a short circuit when triggered, so if any power (ac or dc) is present the GDT will short this out, and can conduct not only the undesired transient, but the power as well. Gas arrestors are commonly used in telecommunication equipment to protect against lightning strikes. Gas discharge tubes have exceptionally low capacitance, and are the only voltage sensitive clamping component that can be used directly on high frequency lines.

• A selenium voltage suppressor is a "clamping" semiconductor similar to a MOV, but it does not clamp as well. However, it usually it has a longer life than a MOV. It is used mostly in high-energy DC circuits, like the exciter field of an alternator. It can dissipate power continuously, and it retains its clamping characteristics throughout the surge event, if properly sized.

• A quarter-wave coaxial surge arrestor is used in RF signal transmission routes. It features a tuned quarter-wavelength short-circuit stub that makes it pass a bandwidth of frequencies, but presents a short to any other signals, especially down towards dc. The bandwidths can be narrow (about +/-5% to +/-10% bandwidth) or wideband (above +/-25% to +/-50% bandwidth). Quarter-wave coax surge arrestors have coaxial terminals, compatible with common coax cable connectors (especially N or 7-16 types). They provide the most rugged available protection for RF signals above 400MHz; much better than gas discharge cells typically used in the universal/broadband coax surge arrestors. Quarter-wave are useful for Telecom, WiFi at 2.4 or 5 GHz but less useful for TV/CaTV. Since a quarter-wave shorts out the line, it is not compatible with systems sending power for a LNB up the coax downlink.

• Crowbar Circuits - A crowbar (circuit) using a zener diode driving the gate of a SCR (silicon controlled rectifier) to latch any high voltages until power is turned off. The zener diode sets the trip voltage, and when exceeded, the SCR will latch on.

• Inductor - An inductor resists sudden changes in current flow. Surge protectors using inductors are sometimes called "series mode" surge protectors, because the inductor is connected in series with the load, rather than diverting current to neutral or ground as with MOVs and diodes. Inductors are usually used in hybrid circuits, in conjunction with the shorting devices above.

• Capacitor - A capacitor resists sudden changes in voltage by absorbing current, and acts as a low pass filter, reducing spikes and noise.

• Resistor - A resistor can even be useful in limiting energy, providing that the voltage and energy absorption are within the limits of the component.
• Overcurrent Protection - A fuse burns out or circuit breaker or polyfuse trips when excessive current flows to a device within the designed time lag and current rating. These can protect both from slower external surges, and by current surges due to the operation or failure of the protective the component itself.

• An iron-poor transformer can transmit AC power similar to a normal iron core transformer (although less efficiently), but will be unable to transmit sudden surges that saturate the small iron core.

• Devices having several outlets with surge protectors described may have all or some outlets protected. The better ones have higher ratings and offer more modes of protection against surges between the phase and neutral and grounding conductors; maybe for telephone and coax cable connections. (The best have insurance policies paying for damages from surges.)

• A UPS of the flywheel type seldom pass surges. Some models that use electrochemical batteries as the energy storage device may absorb spikes much like a capacitor acts as a low pass filter. Almost all battery-backed models also have protectors described above. UPSs of the "On line" topology provide the best protection.

• Power strip

Standards applied for surge protection: IEC 61643-1

• HowStuffWorks: How Surge Protectors Work
• Historical data and discussion on MOV based protectors.